What is the burden of proof in the sanity phase?

Because the defendant has the burden of proof in the sanity phase, "if neither party presents credible evidence on that issue the jury must find him sane. Thus the question on appeal is not so much the substantiality of the evidence favoring the jury's finding as whether the evidence contrary to that finding is of such weight and character that the jury could not reasonably reject it." (People v. Drew (1978) 22 Cal.3d 333, 351, disapproved on other grounds in Skinner, supra, 39 Cal.3d 765.)